1. Field of the Invention
In preferred embodiments, the present invention relate, to use of a biomarker of Alzheimer's disease for diagnostic tests as well as screening assays for agents that may be useful in treating Alzheimer's disease.
2. Description of the Related Art
The search for Alzheimer's disease (AD) biomarkers in body fluids began with measurements of pathogenic molecules, amyloid-β (Aβ) 1-40 and 1-42, in cerebrospinal fluid (CSF) (Blennow K & Hampel H. 2003 Lancet Neurol 2:605-613; Colton C A et al. 2006. J Neuroinflammation 3:27) and plasma, and the search for various markers has continued into genomics, proteomics and metabolomics (Abdi et al. 2006. J Alzheimers Dis 9:293-348.) CSF proteome in Alzheimer's disease has been characterized by several groups and diagnostic utility of a roster of proteins has been shown (reviewed in (Abdi et al., supra)). Plasma proteome shows changes in multiple proteins implicated in the disease pathology, in particular inflammation, but the test is only 56% sensitive and 80% specific (Hye el al. 2006. Brain 129:3042-3050).
CSF- and blood-based tests are gaining entry into clinical use. Low CSF level of Aβ1-42 and a high phosphorylated-τ and total τ levels are widely used in Europe to differentiate mild cognitive impairment (MCI) and early AD from normal aging, depression and alcohol abuse (Blennow K & Hampel H. 2003 Lancet Neurol 2:605-613.). Blood tests are more suitable than CSF tests for screening use. The association of plasma Aβ1-40 and Aβ1-42 levels with dementia has been evaluated in several longitudinal studies (Motter et al 1995. Ann Neurol 38:643-648.; Motter et al. 1995. Ann Neurol 38:643-648; Pomara et al. 2005. Am J Geriatr Psychiatry 13:914-917). The result of a high plasma concentration of Aβ1-40 and a low concentration of Aβ1-42 baseline has been the strongest predictors of dementia in a recent longitudinal study (van Oijen et al. 2006. Lancet Neurol 5:655-660).
Other proposed biomarkers include inflammatory molecules (C1q, interleukin-1 and interleukin-6, transforming growth factor-β, tumor necrosis factor-α, and α-1-chymotrypsin (Mrak and Griffin 2005 J Alzheimers Dis 8:369-375; Mrak and Griffin 2005. J Alzheimers Dis 8:369-375). The presence of inflammatory molecules in body fluids reflects the inflammatory component of AD. Divergent results of inflammatory markers in different studies have been explained by the presence of peripheral immune suppression associated with central neuroinflammatory processes (Mrak and Griffin 2005 J Alzheimers Dis 8:369-375).
Brain amyloidosis of sporadic AD has been attributed to defective Aβ clearance (Motter et al. 1995. Ann Neurol 38:643-648). Peripheral clearance of Aβ by complement C3-adherence to erythrocytes is lower in Alzheimer's disease patients compared to control subjects (Rogers et al. 2006. Neurobiol Aging 27:1733-1739). We have searched for peripheral biomarkers of AD based on the presence of defective phagocytosis of Aβ in patients with sporadic Alzheimer's disease. Macrophages of patients with sporadic Alzheimer's disease generally show defective phagocytosis of Aβ in spite of apparently normal phagocytosis of bacteria, whereas macrophages of a majority of age-matched, cognitively normal persons perform excellent phagocytosis of Aβ (Fiala et al. 2005. J Alzheimers Dis 7:221-232; discussion 255-262). Surprisingly, as shown herein, monocytes freshly isolated from peripheral blood mononuclear cells: (PBMC's) also show robust differences between patients and controls in their interactions with FITC-Aβ. In addition, during phagocytosis of Aβ, control monocytes express several molecules that may serve as biomarkers; in contrast, Alzheimer's disease monocytes are susceptible to apoptosis.
Amyloid-precursor protein (APP) is the precursor protein from which the pathogenic amyloid-beta is cut out. APP is expressed from a single gene in a wide variety of tissue and cell types. At least ten isoforms of the protein have been described, derived by alternative splicing; the 695, 751 and 770 amino acid isoforms appear to be most common. APP695 is exclusive to neurons while the other forms are present in many cell types. The protein has a transmembrane region near the c-terminus, undergoes O-linked and N-linked glycosylation, and contains a Kunitz-type protease inhibitor (KPI) domain in its c-terminal portion”
Sequential proteolytic processing of APP (the 770 amino acid isoform) by β- and γ-secretase after amino acids 671 and 711/713/714, respectively, give rise to the Aβ peptides. The Aβ peptides, known as Aβ x-40, x-42 and x-43 for the number of amino acids they contain, are deposited into the hallmark amyloid plaques of AD. It is these peptides, especially Aβ x-42 and x43, that are thought to be causal in AD.
SEQUENCE LISTING TABLESEQIDPEPTIDESEQUENCENOAmyloidH-Asp-Ala-Glu-Phe-Arg-His-Asp-Ser-1beta 1-40Gly-Tyr-Glu-Val-His-His-Gln-Lys-Leu-Val-Phe-Phe-Ala-Glu-Asp-Val-Gly-Ser-Asn-Lys-Gly-Ala-Ile-Ile-Gly-Leu-Met-Val-Gly-Gly-Val-Val-OH AmyloidH-Asp-Ala-Glu-Phe-Arg-His-Asp-Ser-2beta 1-42Gly-Tyr-Glu-Val-His-His-Gln-Lys-Leu-Val-Phe-Phe-Ala-Glu-Asp-Val-Gly-Ser-Asn-Lys-Gly-Ala-Ile-Ile-Gly-Leu-Met-Val-Gly-Gly-Val-Val-Ile-Ala-OH